The fuel cell is capable of producing electricity without a substantial need for fossil fuel that poses concerns about resource depletion, without noise, and at a high energy recovery rate as compared with other energy-based power generating systems. Great efforts have been made to exploit the fuel cell as a power generating plant of relatively compact size in buildings and factories, with some cells having been commercially implemented. In particular, polymer electrolyte fuel cells (PEFC) can operate at lower temperature than fuel cells of other types. The PEFC then draws attention not only as a device for household co-generation, but also as the replacement power source for internal combustion engines on vehicles because of the minimized corrosion concern regarding the materials of which cell components are made and their ability to discharge relatively high current flow despite low temperature operation. The PEFC is constructed of electrolyte membranes, separators and other components. The separator is generally a plate which is provided with a plurality of parallel channels on one surface or both surfaces. The separator plays the role of conducting the electricity produced at the gas diffusion electrode within the fuel cell to the exterior, discharging water produced within the channels in the course of electricity generation, and securing the channels as a flow path for incoming reaction gas to the fuel cell. Such a fuel cell separator is required to be more compact in size. Since a multiplicity of separators are used in stack, there is a demand for a separator seal material having durability and long term service.
As the separator sealing material, packing materials based on various resins have been under study. Among them, sealing materials based on silicone rubber are often used for their moldability, heat resistance and elasticity. JP-A 11-129396, JP-A 11-309747 and JP-A 2001-58338 disclose liquid silicone rubber compositions of the addition cure type having a good flow and brief cure. These compositions are injection moldable to form seals. However, silicone rubbers obtained by curing the addition cure type compositions are still unsatisfactory in maintaining elasticity over a long term.
JP-A 2002-313373 and JP-A 2003-257455 disclose organopolysiloxane compositions with reduced compression set. For the seal on fuel cell separators, in particular, seal performance in acidic aqueous solution is important. JP-A 2002-309092 discloses how to reduce the compression set in acidic aqueous solution.
For the PEFC, several tens to several hundreds of unit cells wherein thin-film structures having an electrolyte membrane sandwiched between electrodes are alternated with separators and assembled into a stack. In order that the overall stack be compact, it is necessary that separator substrates have a thickness of up to 1 mm and seals formed thereon be as thin as 0.5 mm or less, for example. When such thin-wall seals are formed on separator substrates, the flow and rubber strength of the seal-forming material become important factors. At the same time, acid resistance and compression set are also requisite as the seal. However, JP-A 11-129396 and JP-A 11-309747 cited above merely describe an appropriate viscosity range of 1,000 to 10,000 poise, but refer nowhere to the detail of silicone rubber. While the flow at a high shear is a key for injection molding, these patents do not discuss the viscosity versus shear rate. In our experiment, the silicone resin KE1950-60A/B (Shin-Etsu Chemical Co., Ltd.) used in Examples was found to have a viscosity in excess of 200 Pa-s at a shear rate of 10 s−1, which is not regarded appropriate to form thin-wall seals. JP-A 2001-58338 describes an example in which a liquid silicone rubber material is injection molded to form a seal, but refers nowhere to the composition and flow of the material.
JP-A 2002-313373 discloses a composition comprising fumed silica treated with two different surface treating agents, but refers nowhere to the viscosity and moldability of the composition. JP-A 2003-257455 describes that rubber having improved compression set and strength is obtainable by the combined use of two different alkenyl-containing fluids, but refers nowhere to moldability.